Blue luminescence emitted from ytterbium-doped fiber under infrared excitation was observed, along with well-known green cooperated luminescence. We suggest blue luminescence is due to higher-order cooperated luminescence related three excited ytterbium ions.
An experimental and theoretical investigation of the nonlinear transmission coefficient in a set of Ytterbium-doped silica fibers (YFs) with various concentrations of Yb(3+) ions at continuous-wave 980-nm pumping is reported. An analysis of the obtained experimental data shows that YF transmission coefficient is notably affected by the presence of Yb(3+) - Yb(3+) ion-pairs in the fibers, especially in heavily-doped ones. The last fact is confirmed by the study of the cooperative luminescence and absorption effects in the fibers, where a detailed inspection of their dependence on Yb3+ concentration is presented. The pairs' effect is shown to seriously modify both the nonlinear character of YF transmission coefficient at lambda = 980 nm and Yb(3+) excited-state relaxation. A modeling of the experimental data is performed, which allows to find the coefficients addressing the pairs' effect in each of YFs under study and, as a result, to fit the experimentally measured dependences of YF transmission coefficient on pump power, fiber length, and Yb(3+) concentration.
Cooperative luminescence phenomenon of Yb:YAG crystals was studied. Up-converted blue luminescence was obviously observed in Yb:YAG under excitation in the near-infrared region with a wavelength 940 nm. Experimental evidence was provided by the fact that the intensity of 498 nm luminescence shows a quadratic dependence on excitation power, and the intensity of blue luminescence increased with the increase in Yb ion concentration. It is concluded that this luminescence is due to a cooperative process resulting from a strong interaction of Yb ions. The cooperative luminescence is possiblely assigned to the strong interaction of 4f(13) electrons and neighboring Yb3+ ions in a covalent host lattice such as YAG. Theoretical cooperative luminescence spectrum was calculated.